The present invention relates to a cable reel having a flat cable utilized for electrical connection between a fixed member and a movable member, and more particularly to a cable reel suitable for use with a steering device for an automobile.
A cable reel includes a flat cable constructed under a wound condition with a plurality of turns. The flat cable is fixed at its one end to a fixed member, and is also fixed at the other end to a movable member. The flat cable extends from both ends to the outside of the cable reel. When the movable member is rotated, the flat cable is wound up around the movable member, or it is unwound from the movable member. Thus, the movable member is allowed to be rotated by a predetermined amount. For example, the cable reel is used in a steering device for an automobile.
FIGS. 5 to 7 show a conventional cable reel for an automobile, wherein FIG. 5 is an exploded perspective view of the cable reel; FIG. 6 is a vertical sectional view of the cable reel; and FIG. 7 is a plan view of the cable reel with a part of the movable member omitted. Referring to these drawings, reference numeral 1 designates a movable member centrally formed with a cylindrical inner ring wall 3 having a shaft insertion hole 2. A cylindrical holder 4 is provided around the inner ring wall 3. The holder 4 has a vertical slit 4a and an inside recess 4b. Specifically, the holder 4 is fixed to a top portion of the movable member 1 in such a manner that a predetermined annular space is defined between the inner surface of the holder and the outer surface of the inner ring wall 3.
Reference numeral 5 designates a flat cable formed by laminating a plurality of conductors such as copper foils between two flexible base films formed of polyethylene terephthalate (PET), for example. The flat cable 5 is inserted at its one end portion through the slit 4a of the holder 4, and is received in the recess 4b. The one end portion of the flat cable 4 is longitudinally divided into a pair of leading portions 5a, which are in turn bent back at about 45 degrees to extend along the recess 4b. The pair of leading portions 5a in the recess 4b are overlapped with each other and bonded in a snap fashion to each other. The leading portions 5a thus bonded are held between first and second holders 6 and 7 fixed to the top portion of the movable member 1. Under the condition, a tapering portion 6a of the first stopper 6 is inserted into the bent portions of the leading portions 5a.
Reference numeral 8 designates a fixed member formed with a central hole 9 for rotatably inserting the inner ring wall 3 of the movable member 1. The fixed member 8 is formed at its outer circumference with an outer ring wall 10 for surrounding the flat cable 5. The outer ring wall 10 is formed with a guide portion 11 projecting obliquely downwardly therefrom for guiding the other end portion of the flat cable 5. Specifically, the other end portion of the flat cable 5 is bent back at about 45 degrees at the guide portion 11, and is led out of the fixed portion 8 through the guide portion 11. The outer ring wall 8 is further provided with a positioning mechanism 13 having a gear 12 adapted to engage a projection 14 formed at the outer circumference of the movable member 1. Every time the movable member 1 is rotated 360 degrees, the gear 12 is brought into engagement with the projection 14 and is therefore rotated by a given angle.
The cable reel thus constructed is actually used in such a manner that the movable member 1 is mounted on a steering wheel side of a steering device for an automobile and the fixed member 8 is mounted on a bearing side of a steering shaft. When the steering wheel (the movable member 1) is rotated in one direction, the flat cable 5 is wound up on the holder 4 (the inner ring wall) of the movable member 1. Reversely, when the steering wheel is rotated in the other direction, the flat cable 5 is unwound on the outer ring wall 10 of the fixed member 8. Under any conditions, electrical connection between the movable member 1 and the fixed member 8 is ensured via the flat cable 5. Further, every time the movable member 1 is rotated 360 degrees, the gear 12 is brought into engagement with the projection 14 to rotate by a given angle. Therefore, a specific tooth of the gear 12 and a case of the positioning mechanism are marked respectively so as to eliminate a difference in rotational amount in opposite directions of the flat cable 5, thus preventing breakage and buckling of the flat cable 5.
However, nevertheless the positioning mechanism is provided to properly position the movable member 1 and the fixed member 8, it is hard to completely eliminate erroneous mounting of the cable reel. That is, there is a possibility that the movable member 1 is mounted at a position rotationally offset from a neutral position in either direction. In the case that the movable member 1 is erroneously mounted in such a manner that a rotational amount of the movable member 1 in a winding direction of the flat cable 5 is less, when the movable member 1 is rotated by a predetermined amount or more, a tension in the winding direction is applied to the flat cable 5. Therefore, a cutter may be provided on the fixed member 8 at a position opposed to the bent portion of the flat cable 5, so as to forcibly cut the flat cable 5 by the cutter and electrically detect the erroneous mounting.
Reversely in the case that the movable member 1 is erroneously mounted in such a manner that the rotational amount of the movable member 1 in an unwinding direction is less, even when the movable member 1 is rotated by a predetermined amount or more, the flat cable 5 is inverted at its inner circumferential end portion as shown in FIGS. 8(a), 8(b) and 8(c). In this manner, the excessive rotation of the movable member 1 cannot be avoided, and the erroneous mounting cannot be therefore electrically detected. Further, once the flat cable 5 is inverted as mentioned above, such inversion will be readily repeated upon subsequent rotation of the movable member 1, causing sudden disconnection of the flat cable 5 in use.
It is proposed that a reinforcing film is fixed to the inner ring wall of the movable member along the inner circumferential end of the flat cable so as to suppress the inversion of the flat cable by the flexural rigidity of the reinforcing film (Japanese Utility Model Laid-Open Publication No. 62-168581). According to this technique, the reinforcing film must be flexed so as not to hinder the winding operation of the flat cable. Therefore, it is impossible to completely eliminate the inversion of the flat cable owing to the flexural rigidity of the reinforcing film only. Consequently, the problem of disconnection of the flat cable due to the erroneous mounting has not yet been solved in the prior art.